interfaces/apparmor: add missing call to dirs.SetRootDir

[zyga]

This patch corrects a test that was failing on openSUSE. On Ubuntu it
was passing by accident because /var/cache/apparmor/$something did not
exist (the code treats file-not-found as an OK condition). On openSUSE
the permissions prevent non-root users from traversing
/var/cache/apparmor so the test failed with permission error.

In both cases what was missing was a call to dirs.SetRootDir(), to
isolate the test from the real system.

Signed-off-by: Zygmunt Krynicki me@zygoon.pl

[codecov-io]

Codecov Report

Merging #3813 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3813      +/-   ##
==========================================
+ Coverage   75.81%   75.82%   +<.01%     
==========================================
  Files         402      402              
  Lines       34793    34793              
==========================================
+ Hits        26380    26382       +2     
+ Misses       6540     6538       -2     
  Partials     1873     1873

Impacted Files	Coverage Δ
overlord/snapstate/snapstate.go	80.4% <0%> (-0.27%)	⬇️
interfaces/sorting.go	100% <0%> (+2.56%)	⬆️
wrappers/binaries.go	79.54% <0%> (+6.81%)	⬆️

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[ogra1]

seems to do what it says on the tin ...
+1